The present invention relates to the sphere of reduction of the pressure drops of a fluid circulating in a pipe.
Document WO-84/03,867 is aimed to reduce the drag forces generated by the relative velocity of a fluid on an object by providing the surface of the object with grooves. French patent applications No. 02/02,598 and No. 02/02,599 present a method of manufacturing grooves on the inner surface of a pipe.
The petroleum industry uses pipes, commonly called  less than  less than pipelines greater than  greater than  or  less than  less than gas pipelines greater than  greater than , for conveying gas or petroleum effluents over long distances. In order to reduce the number of recompression stations placed along these pipes, it is interesting to be able to reduce the pressure drops generated as the fluid flows through these pipes.
There are also pipes whose walls are deformable in order to reduce the pressure drops generated by the fluid flow. The deformable walls are provided with active systems (exciting systems) or passive systems.
The present invention provides another technique for reducing the pressure drops generated as a fluid flows through a pipe. It provides passive control of the flow by coating the inner wall of the pipe with a layer of a porous and permeable material.
In general terms, the invention relates to a method for decreasing the pressure drops generated by the flow of a fluid in a tube, the method comprising the following stages:
putting a layer of a porous material onto the inner surface of the tube impermeable to the fluid, and
selecting the material having a permeability which is such that said fluid circulates in said layer.
According to the invention, at least one of the characteristics of said material layer can be determined so as to reduce the pressure drops generated by the fluid flow in the pipe, the characteristics being the porosity of the material, the permeability of the material and the thickness of the layer. The porosity of the material can range between 1% and 50%, preferably between 5% and 35%. The permeability of the material can range between 0.005 D and 5 D, preferably between 0.1 D and 1 D. The thickness of the layer can be determined according to the viscous layer of the fluid flow.
The invention also relates to a pipe for conveying a fluid. The pipe comprises a tube impermeable to the fluid conveyed and a layer of a porous material covering the inner wall of the tube. The permeability of the material is such that the fluid circulates in the layer.
According to the invention, the porosity of the material can range between 1% and 50%, preferably between 5% and 35%. The permeability of the material can range between 0.005 D and 5 D, preferably between 0.1 D and 1 D. The thickness of the layer can be greater than the thickness of the viscous layer generated by the fluid flow on the porous and permeable material. The layer of porous and permeable material can comprise grooves arranged in the direction of flow of said fluid and/or the layer of porous and permeable material can be deformable.
The method and/or the pipe according to the invention can be used for natural gas transportation.
The invention allows passive control of the flow of a fluid in a pipe, i.e. the invention requires no exciting systems, instruments or exterior energy supply for producing a pressure drop reduction. It allows to reduce by up to 30% the value of the pressure drops of a fluid on turbulent flow in a pipe. The presence of a layer of a porous and permeable material on the inner wall of the pipe creates a buffer zone which changes the characteristics of the flow in the vicinity of the wall. In particular, the viscous layer allows a velocity increase in the vicinity of the porous wall without any pressure drop increase. Furthermore, the layer of permeable and porous material allows the transition from a laminar flow to a turbulent flow to be delayed. Besides, the invention allows the swirling-flow mechanisms in the viscous layer to be controlled.